1. Field of the Invention
This invention relates to an anisotropic electrically conductive film obtained on dispersing electrically conductive particles in a thermosetting acrylic resin composition and which exhibits a high adhesion strength, and to a connection structure that makes use of the same.
The present application claims priority rights based on the Japanese Patent Application 2007-115929, filed in Japan on Apr. 25, 2007. This patent application of the senior filing date is to be incorporated herein by reference.
2. Description of Related Art
In the field of the packaging technology, an anisotropic electrically conductive film has now come into widespread use because the film allows for a simpler connection process and also is lead-free. As this anisotropic electrically conductive film, such a film composed of a thermosetting epoxy resin composition was so far used in view of connection reliability and resistance against heat. Recently, the attention is focused on an anisotropic electrically conductive film formed of a thermosetting acrylic resin composition in order to meet the demand for shorter connection time and for a lower temperature.
With this type of the anisotropic electrically conductive film, those azo-based initiators and peroxide-based initiators, as curing agents, which are low in decomposition and activation temperatures, may selectively be used, as a result of which it is possible to carry out a packaging operation under heating and pressure application at a lower temperature of the range of 140 to 160° C. for a shorter time duration less than 10 sec.
Meanwhile, if an anisotropic electrically conductive film formed of an acrylic resin composition is compared to an anisotropic electrically conductive film formed of an epoxy resin composition, the former has deficiencies that are connected with low adhesion strength, such as a defect in the connection itself and in low conduction reliability subsequent to connection.
To solve this problem, such an anisotropic electrically conductive film in which a hydroxyl-containing resin, with a molecular weight not lower than 10000, is contained in the acrylic acid composition, and in which a phosphorus-containing acrylic ester is selectively used as the acrylic ingredient (thermo-settable ingredient), is now being searched as disclosed in Japanese Patent No. 3587859, for instance.
With this anisotropic electrically conductive film disclosed in the Japanese Patent No. 3587859, the adhesion strength with respect to a support material such as an electronic substrate may be expected to be elevated owing to the presence of the hydroxyl groups. However, in actual packaging operations, it is requested to raise the adhesion strength in a state the stress generated in the support material is relieved, that is, in a state the support material is not warped. It is not possible with the known film to satisfy the requirements which are contradictory to each other, that is, the requirement for achieving the elevated adhesion strength and the requirement for realization of a stress relieved state, at the same time.
That is, to meet these requirements which are contradictory to each other, it is felt to be necessary that a polymer ingredient of as high a molecular weight as possible is used as a resin material other than a ingredient that is responsible for contraction on curing (thermo-settable ingredient). However, the anisotropic electrically conductive film disclosed in Patent Publication 1 is not of a sufficiently high molecular weight. Specifically, there is a statement in Patent Publication 1 that a polymer material with a weight averaged molecular weight not less than 1000000 is inferior in its dissolution performance and is difficult to mix in the composition. While there is also a statement in Patent Publication 1 that the acrylic rubber is mixed in the composition to relax the stress as well as to elevate the force of cohesion of the adhesive, the molecular weight of the acrylic rubber is about 200000 at most.
That is, with the thermosetting acrylic resin composition in the past, it is difficult to elevate the adhesion strength in a stress-relieved state.